Ethylene-α-olefin copolymers have been molded into films, sheets, bottles, and so on by various molding processes and have been used for various applications such as food wrapping materials.
Among ethylene-α-olefin copolymers, copolymers produced by polymerization using metallocene catalysts are known to be superior in mechanical strength such as impact strength and tensile strength. Therefore, since the use of such copolymers can reduce the thickness of articles formed from ethylene-α-olefin copolymers produced by polymerization using catalysts other than metallocene catalysts while maintaining the mechanical strength thereof, reduction in the weight or the cost of articles can be expected; therefore, the use of such copolymers for various applications is being studied. However, since ethylene-α-olefin copolymers produced by polymerization using conventional metallocene catalysts are high in extrusion load in extrusion processing and low in melt tension, improvement in molding processability thereof has been desired. Moreover, ethylene-α-olefin copolymers obtained by using metallocene catalysts generally are narrow in comonomer composition distribution and is unified in melting temperature of crystals and, therefore, tend to be inferior in secondary processability, e.g., narrow in the range of temperature at which heat-sealing can be achieved.
As a countermeasure thereto, novel metallocene catalysts have recently been studied and ethylene-α-olefin copolymers with improved molding processability produced by polymerization using such catalysts have been proposed. For example, patent literature 1 has disclosed an ethylene-α-olefin copolymer produced by polymerization using a metallocene catalyst composed of a transition metal compound having a ligand in which two groups having a cyclopentadiene type anion skeleton are linked via a bridging group, a transition metal compound having two groups having a substituted cyclopentadiene type anion skeleton, the two groups not being linked to each other, and an activating promoter component. Patent literature 2 has disclosed an ethylene-α-olefin copolymer produced by polymerization using a promoter component prepared by bringing silica, hexamethyldisilazane, diethylzinc, pentafluorophenol, and water into contact with each other, and a metallocene catalyst composed of triisobutylaluminum and racemic ethylenebis(1-indenyl)zirconium diphenoxide. Patent literature 3 has disclosed an ethylene-α-olefin copolymer produced by polymerization using a metallocene catalyst comprising a combination of a transition metal compound having a ligand in which two groups having a cyclopentadiene type anion skeleton are linked via abridging group and a transition metal compound having a ligand in which a group having a cyclopentadiene type anion skeleton and a group having a fluorenyl type anion skeleton are linked via a bridging group, and a support as a promoter, the support having been prepared by loading methylalumoxane onto porous silica.    Patent Literature 1: JP 2003-96125 A    Patent Literature 2: JP 2004-149761 A    Patent Literature 3: JP 2006-233206 A
However, the ethylene-α-olefin copolymers disclosed in patent literatures 1 and 2 have been reduced in extrusion load in molding processing in comparison to ethylene-α-olefin copolymers produced by polymerization using conventional metallocene catalysts, but they are required for further reduction in extrusion load; the ethylene-α-olefin copolymer disclosed in patent literature 3 was not satisfactory in a strain hardening property and take-up property at the time of processing because of insufficient introduction of long chain branches into the main chain.